The present invention relates to a plugging meter for automatically measuring the density or concentration of impurities contained in a liquid metal such as liquid sodium and the like. More particularly, the present invention is concerned with a plugging meter capable of automatically and accurately observing a multi-plugging phenomenon taking place in a system composed of structural members of ferritic stainless steel and austenite stainless steel, such as a sodium-heated steam generator.
As is well known, liquid metals such as liquid sodium have a large heat-transfer coefficient, and remain in liquid phase to a sufficiently high temperature. In addition, they are less likely to be damaged by radioactive rays. For these reasons, the liquid metals are advantageously used as coolants for power sources which operate at a high temperature, such as a nuclear reactor and the like.
Impurities in liquid sodium (chiefly Na.sub.2 O, NaOH, NaH, Na.sub.2 CO.sub.3 and the like) must be removed by a cold trap or the like, even when they are present in extremely small amounts, because they may cause corrosion and/or clogging of the sodium-carrying apparatus.
A plugging meter has been developed as a means for detecting the concentration or density of the impurities in liquid metals, and has been successfully used in managing the purity of liquid sodium.
However, the initial operating period of sodium loops or in sodium systems employing ferritic stainless steel, a multi-plugging phenomenon is often observed, which makes automatic measurement difficult.
The plugging meter has an orifice installed in the path of the liquid sodium, and makes use of a plugging of the orifice by the deposition of impurities which takes place as the temperature of the liquid sodium is lowered to a certain temperature. Thus, the concentration or density of the impurities can be determined by the reduction of flow rate, in relation to the temperature around the orifice.
All of the plugging meters proposed up to now are based on this principle, but have various constructions. The plugging meter which is most relevant to the present invention has a following construction.
A cooling passage provided with cooling equipment and a by-pass passage in combination therewith constitute parallel liquid sodium paths which are connected between an inlet pipe having a first flow meter and an outlet pipe.
The liquid sodium having entered the inlet pipe then flows through both the cooling passage and the by-pass passage, and then the parallel flows are formed and discharged through the outlet pipe. A plugging orifice section is provided in the cooling passage, near the downstream end or outlet port thereof. The orifice section is provided with a thermocouple for measuring the sodium temperature.
Also, the cooling passage has a second flow meter for measuring the sodium flow rate in the plugging orifice section. The output signals from above two flow meters are processed by a divider, which produces a signal representative of the ratio of the flow rates. The temperature of the orifice is controlled by means of a blower and an associated damper of the cooling equipment.
For manual measuring by this plugging meter, at first the blower is started and then the damper is gradually opened manually, while watching the temperature of the plugging orifice, so as to gradually lower the temperature. After a certain period of the constant flow-rate ratio signal, the plugging starts to take place, which can be detected by a decrease of the signal. In case of a double plugging pattern, at first a first plugging is observed and, after a subsequent temporary lull, a second plugging comes to be observed as the temperature is further lowered. The substances causing plugging, as well as their concentrations, can be determined from the respective plugging temperatures.
Then, the damper is fully closed and the fan is stopped to allow the temperature of the plugging orifice to rise. The temperature is then raised up to the starting temperature, for unplugging the orifice.
The plugging meter of the kind described can be controlled for an automatic measurement, when combined with a suitable automatic controller as described below.
Namely, an automatic controller is provided which performs a comparison of the input signal representative of the flow-rate ratio from the divider with a given value. Usually, this given value is selected to be a signal value equal to the flow-rate ratio exhibited when there is a slight deposition of the impurities at the plugging orifice. The automatic controller acts to increase the degree of opening of the damper to lower the temperature of the liquid sodium, so as to promote the deposition of the impurities, when the measured flow-rate ratio is greater than the given value, while, when the measured flow-rate ratio is smaller than the given value, the controller acts to increase the degree of opening of the damper, so that the sodium temperature will be raised to enhance the dissolution of the impurities.
Thus the controller serves to maintain an equilibrium of deposition and dissolution of impurities at the plugging orifice, and the sodium temperature at this state is detected as the plugging temperature.
It will be seen that the multi-plugging phenomenon can be detected by a manual measurement only by a very troublesome measuring operations. On the other hand, the automatic measurement system can perform the measurement for only one plugging substance. Further, in the latter case, the plugging temperature is difficult to establish when the plugging substance is one which is only slightly dissolved by the liquid sodium, so that the measurement is made difficult.